AFM question
Posted: Fri Sep 27, 2013 3:23 am
I have a question about the old style Vane air flow meters.
It's call an air flow meter not mass flow meter. However, will the temperature of the air change the reading any, assuming the same cfm flowing through it? The reason I wonder is that cold air flowing through the meter will have more mass than warmer air, so I assume it can push "harder" on the vane and thus hold it open more and thus a larger voltage output for colder air vs warmer air. Does that sound correct? For example: 1000cfm of an extremely dense fluid such as peanut butter (creamy not crunchy) should push the vane open further than 1000cfm of 200deg air. Therefore it's more of an air MASS sensor than an air FLOW sensor. I keep going back and forth on which it is.
These systems also have an air temp sensor in them that the ecu definitely uses to adjust fuel, so my above assumption may be partially wrong or totally wrong. I think there are these possibilities:
Given a fixed amount of CFM of air flowing, the AFM voltage output is proportional to:
1. ONLY air volume flow. It's a true volume flow measurement device regardless of IAT or density of fluid flowing.
2. mostly air volume flow, with a little extra vane movement for colder air but not enough to call it a true mass sensor.
3. air mass flow as the vane movement is primarily a function of the mass of air flowing past, not the volume.
Len
It's call an air flow meter not mass flow meter. However, will the temperature of the air change the reading any, assuming the same cfm flowing through it? The reason I wonder is that cold air flowing through the meter will have more mass than warmer air, so I assume it can push "harder" on the vane and thus hold it open more and thus a larger voltage output for colder air vs warmer air. Does that sound correct? For example: 1000cfm of an extremely dense fluid such as peanut butter (creamy not crunchy) should push the vane open further than 1000cfm of 200deg air. Therefore it's more of an air MASS sensor than an air FLOW sensor. I keep going back and forth on which it is.
These systems also have an air temp sensor in them that the ecu definitely uses to adjust fuel, so my above assumption may be partially wrong or totally wrong. I think there are these possibilities:
Given a fixed amount of CFM of air flowing, the AFM voltage output is proportional to:
1. ONLY air volume flow. It's a true volume flow measurement device regardless of IAT or density of fluid flowing.
2. mostly air volume flow, with a little extra vane movement for colder air but not enough to call it a true mass sensor.
3. air mass flow as the vane movement is primarily a function of the mass of air flowing past, not the volume.
Len